Since the inception of Chip Scale Packages (“CSPs”) and flip chips, the increasing demand for smaller electronic component packages has intensified the focus on related manufacturing processes to find methods to increase their reliability. The capillary underfill process was developed to provide the necessary thermo-mechanical reinforcement to mitigate the coefficient thermal expansion (“CTE”) mismatch, thus preventing early failure of the devices. The CTE is the difference in expansion and contraction between a board and a semiconductor chip, which weakens the solder joints and induces stress on the chip and on the board. Underfill material, typically epoxy adhesive, absorbs the stress induced by the CTE mismatch, secures and stabilizes the chip to the board. FIGS. 1A-1D illustrate a typical prior art capillary underfill process for flip-chip assembly 100. As shown in FIG. 1A, a chip 103 having conductive bumps 102 is placed using the placement nozzle 110 on a printed circuit board (“PCB”) 104, such that the bumps 102 are aligned over the conductive pads 105 of the PCB 104. Prior to alignment, to help soldering conductive bumps 102 of a chip 103 to the PCB 104, a flux material is applied to conductive pads 105 of a PCB 104. The bumps 102 made of eutectic alloy provide electrical connection between a chip circuitry and the conductive pads 105 of the PCB 104. Next, reflow soldering is applied, bumps 102 are melted to form solder joints 106 to conductive pads 105 of the PCB 104, as shown in FIG. 1B, enabling an electrical connection between the circuitry of the chip 103 and the PCB 104. Further, as shown in FIG. 1C, a capillary underfill material 107 is dispensed in the space between the chip 103 and the PCB 104, which are already bonded together by solder joints 106. Prior to the capillary underfill material application, the assembly 100 may be cleaned to remove residuals of the soldering process and may be preheated to improve the flow characteristics of the capillary underfill. Thereafter, as shown in FIG. 1D, the capillary underfill material 107 is dispensed along one or two edges of the chip 103 and is allowed to flow, by capillary action, between the chip 103 and the PCB 104 through a small gap. The capillary underfill material helps to reduce strain induced on the solder joints, on the chip and on the PCB by CTE mismatch. Next, the capillary underfill material 107 is cured in a batch or re-flow oven at elevated temperature over an extended period of time to form polymerized adhesive protective layer 109 between the chip 103 and the PCB 104, as shown in FIG. 1D. Typical capillary underfill materials include epoxy resin and amine-based, anhydride-based, or phenol-based epoxy curing agents.
The capillary underfill process, however, becomes a time consuming and costly part of the manufacturing process. The process requires labor intensive steps such as application of a flux material to help soldering the bumps of the chip to the pads of the PCB, cleaning the assembly after soldering, dispensing capillary underfill, and then curing the capillary underfill. Capillary underfill process is used to protect structures after the chip and PCB are bonded together with solder joints. However, reflow soldering and subsequent cooling down of the assembly to form solder joints before applying the underfill material introduces additional stress to the chip, solder joints, and the PCB that can lead to premature device failure.
No-flow underfill process eliminates lengthy capillary flow times and combines fluxing, soldering and underfilling together. Solder joints are reflowed while fluxing and polymerization of the no-flow underfill takes place, because fluxing agent is already incorporated into the no-flow underfill material. Currently, because anhydrides and phenol-based materials do not react with free organic acids, they are used together with a free organic acid as a fluxing agent to form the non-flow underfill. Amine-based materials are not utilized to form the non-flow underfill, because a free reactive acid of the fluxing agent reacts with amine-based materials that prematurely destroys the fluxing agent. Unfortunately, anhydride and phenol based no-flow underfill materials have poor reliability, in particular at high temperatures, for example, fail in highly accelerated stress tests (“HAST”) and in temperature cycling tests from −55° C. to 125° C. (“TCB”) that compromises their ability to protect the devices.